Sp3 Proteins Negatively Regulate β Myosin Heavy Chain Gene Expression during Skeletal Muscle Inactivity

Abstract

ABSTRACT In adult skeletal muscle, β myosin heavy chain (βMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression is down-regulated in response to muscle inactivity. Little is known about the signaling pathways and transcription factors that mediate this important functional response. This study demonstrates that increased binding of Sp3 to GC-rich elements in theβ MyHC promoter is a critical event in down-regulation ofβ MyHC gene expression under non-weight-bearing conditions. Conversely, binding of Sp3 to these elements decreased while Sp1 binding increased with nuclear extracts from plantaris muscle exposed to mechanical overload, a stimulus that increases βMyHC gene expression. In addition, these experiments revealed the existence of an Sp4-DNA binding complex when using adult skeletal muscle nuclear extract was used but not when nuclear extracts from cultured myotubes were used. Sp3 proteins are competitive inhibitors of Sp1-mediatedβ MyHC reporter gene transactivation in both Drosophila SL-2 and mouse C2C12 myotubes. Sp4 is a weak activator of βMyHC gene expression in SL-2 cells, which lack endogenous Sp1 activity, but does not activate βMyHC gene expression in C2C12 myotubes, which have high levels of Sp1. These results suggest that competitive binding of Sp family proteins regulate βMyHC gene transcription in response to altered neuromuscular activity.